A measuring light beam, the light of which comprises the spectral lines of the looked-for element, among them an absorption line, is directed through such a cloud of atoms or flame. The atoms of the looked-for element absorb such light very strongly, while this measuring light beam theoretically is not influenced by the atoms of the remaining elements of the sample. The lines of the measuring light beam do not correspond to the absorption lines of the other sample elements.
These ideal conditions, however, often are not present. An unspecific absorption, i.e. not caused by the atoms of the looked-for element, occurs, such absorption being called background absorption. This background absorption is due to the elements of the sample not atomized, namely solid particles or molecules absorbing in a wide band. With highly sensitive measurements, this background absorption is a factor not to be neglected and must be compensated.
For this purpose it is known from No. DE-C- 1,964,469 to frequency modulate the measuring light beam, which originates from a single radiation source formed as line emitter. According to No. DE-C- 1,964,469 this frequency modulation is effected by using the Zeeman effect. If a magnetic field is applied to the line emitting light source, for example a hollow cathode lamp or a high-frequency discharge lamp, a change of the energy level permitted by quantum mechanics of the emitting atoms occurs in the light source. Consequently each emitted spectral line is split into a plurality of lines. In a magnetic field applied transversely to the direction of the measuring light beam, splitting occurs into a central component, which is unshifted relative to the original spectral line but polarized parallel to the direction of the magnetic field, and further into components, which are symmetrically arranged to the central component with respect to wavelengths and are polarized perpendicularly to the direction of the magnetic field. In an arrangement, in which the magnetic field points in the direction of the measuring light beam, the central unshifted component does not appear.
From No. No. DE-A-2,165,106 a method of background compensation in the atomic absorption spectroscopy is known, with which also a frequency modulation of the spectral line emitted from the light source is effected for the purpose of background compensation. No. DE-A-2,156,106 mentions also the Doppler effect as a means for frequency modulating the emission line. But how frequency modulation by means of the Doppler effect is to be realized in practice, is not said in No. DE-A2,156,106. Also this printed publication describes line splitting by means of the Zeeman effect as preferred embodiment.
From No. AU-A-251,689 it is known to scan the profile of a spectral line by means of atomic absorption spectroscopy (AAS) or atomic fluorescence spectroscopy (AFS) by shifting a very narrow emission or absorption line relative to an absorption or emission line respectively, of greater width to be analysed. According to the teaching of No. AU-A-251,689 this frequency shift can be effected by using the Doppler effect, the Zeeman effect or the Stark effect. In an embodiment described in No. U-A-251,689 the partial pressure of a looked-for element in a sample is measured by scanning the absorption lines by applying the Zeeman effect, either to the light source or to the sample and recording two-dimensionally the absorption as function of wavelength, integration being effected throughout the whole wavelength profile for determining the quantity or the partial pressure of the looked-for element.
Also this printed publication does not show in detail how the Doppler effect is to be used for wavelength modulation.